Environmental considerations have become important in a variety of fields in recent years. Reduction in carbon dioxide, in particular, has become an urgent issue, and efforts are being made to increase energy efficiency in domestic industrial fields and in transportation fields, including automobiles, as well as in styles of consumption in general.
For example, in systems of industrial machinery that require large work energy, such as injection molding machines, machine tools and press working machines, there are often employed hydraulic systems capable of converting the pressure energy of hydraulic pumps to kinetic energy. The need for energy savings is also high in such hydraulic systems, and energy-efficiency countermeasures are being sought for hydraulic oils used as hydraulic system pressure media, with the aim of achieving lower viscosity and a higher viscosity index, but reduced viscosity has led to problems such as reduced abrasion resistance and seizing at sliding parts.
Recently, loads on sliding sections have been increasing due to trends toward smaller sizes, higher speeds and greater fuel efficiency and energy savings in mechanical systems, creating a demand for lubricant oils with more excellent lubricity, including wear resistance. The use of oxygen-containing synthetic oils such as esters and ethers has been increasing to meet this demand.
From the viewpoint of environmental pollution, on the other hand, biodegradable lubricant oils such as animal or vegetable oils or esters with specific structures are being increasingly employed as environmentally friendly base materials. It is therefore expected that lubricant oils with oxygen-containing compounds as base oils, exhibiting characteristics not found in hydrocarbon oils such as mineral oils, will become even more prevalent in the future.
The ideal characteristics for a lubricant oil are low frictional loss and low wear such as fretting wear, both at high speeds and at low speeds. That is, a lubricant oil is desired to have low frictional loss and reduced wear. It is therefore desirable to have low frictional loss and minimal wear both during periods of high contact surface speeds such as during high-speed rotation, and during periods of high torque at low speeds.
In the case of polar oxygen-containing compound-based lubricant oils, however, wear-resistance agents used in common hydrocarbon-based lubricant oils such as mineral oils have affinity with polar base oils, and therefore the concentration of the wear-resistance agent at sliding sections is low resulting in a poor lubricity-improving effect, such that further development of effective wear-resistance agents in oxygen-containing compound-based lubricant oils such as esters is desired.
Polyol ester- and ether-based oxygen-containing compounds that exhibit compatibility with hydrofluorocarbon refrigerants are used as base oils in the field of refrigerating machine oils, and it has been proposed to add 3,4,5-trihydroxybenzoic acid esters to such base oils, in order to inhibit elution of lead in refrigerating air conditioners that comprise lead-containing bearings (Patent document 1).